The present invention relates to a variable bitrate video coding method including an iterative process that comprises a first analysis step, for coding a bitstream corresponding to a picture sequence with a constant quantization stepsize, and a second prediction step, for predicting the quantization stepsize which must then be used to code said bitstream according to a predetermined target bit rate, and is followed by a final control step, for adjusting the stepsize with respect to said target bit rate. The invention also relates to a corresponding video coder allowing to carry out said method.
As described in the document  less than  less than  MPEG Video coding: a basic tutorial introduction  greater than  greater than , S. R. Ely, BBC Research and Development Report, BBC-RD-1996/3, pp.1-10, MPEG activities started in 1988 with the aim of defining standards for digital compression of video and audio signals. The first goal was to define a video algorithm for digital storage media such as the CD-ROM (Compact Disc Read-Only Memory), but the resulting standard was also applied in the Interactive CD system (CD-I). Allowing transmission and storage of picture data at bit rates in the range of 1 to 15 Mbits/s, this standard is based on a data compression achieved by using a block-based motion compensation for the reduction of the temporal redundancy and a discrete cosine transformation (DCT) for the reduction of the spatial redundancy.
With conventional CD standards such as CD-I and CD-ROM, the transmission bit rate is fixed and pictures can therefore only be coded at a constant bit rate. New standards such as the Digital Versatile Disc (DVD) allow for transmission of data at a variable bit rate (VBR): complex scenes can be coded at a higher bitrate than scenes containing less information, in order to maintain a constant quality.
An object of the invention is to propose a VBR video coding method allowing to obtain such a constant quality of the coded sequence, with a minimal bit rate of the encoder output bitstream.
To this end the invention relates to a VBR video coding method such as defined in the preamble of the description and which is moreover characterized in that it comprises, between the analysis and prediction steps of one iteration, a picture re-arrangement step.
Such a picture re-arrangement step preferably comprises in series a first scene change detection sub-step, a second allocation sub-step, and a third optimum placement sub-step.
In a preferred implementation, said scene change detection sub-step comprises a correlation operation, carried out between succeeding pictures of the sequence, and a decision operation, for indicating the possible occurrence of a scene change.
Another object of the invention is to propose a VBR video coder allowing to carry out said coding method.
To this end the invention relates to a variable bit rate video coder comprising a first coding branch, a second prediction branch, and a control circuit provided for carrying out the implementation of the following operations:
a coding operation of a bitstream corresponding to a picture sequence with a constant quantization stepsize;
a prediction operation, for an estimation of the quantization stepsize allowing to code said bitstream according to a specified target bitrate;
at least one repetition of said operations;
a final controlling operation, for adjusting the stepsize with respect to said target bitrate;
characterized in that said control circuit is provided for implementing, between the first coding operation and the first prediction operation, a picture re-arrangement operation.